1. Field of the Invention
The present invention relates to a method for surface treatment of a sliding portion, and in particular, to a method for surface treatment of a sliding portion, in which lubricativeness is given to the sliding portion of a tool, a die, a piston of an engine, a bearing, a shaft, and in addition hereto, an article including a portion (a sliding portion) sliding on another member as an object to be treated.
2. Description of the Prior Art
In many cases, fluid lubricant such as oil or grease is used in lubricating the sliding portion. However, responding to the case where the fluid lubricant cannot be used for reason of the design of the sliding portion, or the case where fluid and adsorptive air are evaporated and desorbed due to restrictions in the use environment, for example, like the case of use in vacuum, and further, and by the social demand for reducing as much as possible the use of fluid lubricant, accompanied by the recent trend of more sensitive reaction to environmental problems or the like, various solid lubricants have been proposed.
As one example of such solid lubricants, graphite(C), molybdenum disulfide (MoS2), tungsten disulfide (WS2), boron nitride (BN), or the like is used.
In addition, there is a method of forming a wear resistance layer on the sliding portion by use of the solid lubricant, by injecting particles of the solid lubricant onto the surface of the object to be treated at a predetermined injection pressure or speed or more to diffuse and penetrate elements in the solid lubricant composition into the surface thereof (refer to Japanese Patent KOKAI (LOPI) No. 11-131257).
In addition, there has been proposed a method in which solid lubricant powder and soft metal are mechanically contacted with each other for forming solid lubricant-containing soft metal powder, and a low-friction layer is formed by causing this solid lubricant-containing soft metal to contact with the surface of a metal member by use of mechanical means such as a shot blast, a barrel, and a ball mill (Japanese Patent KOKAI (LOPI) No. 2000-282259).
In a shot blast or shot peening of injecting the injection particles together with compressed air, when the compressed air is injected toward the object to be treated with an injection nozzle as shown in FIG. 1 (B), the compressed air which has collided with the surface of the object to be treated is reflected back by the object to be treated, thereby causing a layer made of the compressed air to be formed on the surface of the object to be treated, and together therewith the compressed air which has been interrupted in going straight to the object to be treated to change its flow into the direction along the surface of the object to be treated.
Therefore, in a case where the injection are smaller and lighter in relative relation to the injection pressure or the like of the compressed air, the injection particles get caught up in the flow of the compressed air, and the injection particles together with the compressed air are forced to change the flying direction thereof. As a result, the amount of injection particles colliding with the surface of the object to be treated is reduced. Further, collision energy of the injection particles is reduced due to the change in the flying direction as described above, or due to the layer of the compressed air formed on the surface of the object to be treated, and it becomes impossible to efficiently perform the diffusion/penetration of the injection particles into the object to be treated, and in turn, the formation of the wear resistance layer.
Therefore, as injection particles used in the shot blast or the shot peening, the injection particles having a particle size which enables the injection particles to effectively collide with the surface of the object to be treated is selected in light of the degree and purpose of working or in the relative relation to the working condition such as the injection pressure, and in the case where the injection particles once injected are used again, the fragmentized particles are removed out of the collected injection particles, thereby to make an adjustment so that their size is in a predetermined range.
By the way, graphite(C), molybdenum disulfide (MoS2), tungsten disulfide (WS2), or boron nitride (BN) exemplified as the solid lubricant is a layer-structure solid lubricant. In explanation of graphite as one example, graphite is of a hexagonal plate-crystal layered structure, wherein a carbon-carbon bond is formed on the surface of each layer constituting the layered structure by strong covalent bonding, whereas an inter-layer bond is formed by a relatively weak force. And the sliding occurs between the layers when a load is applied to the layered structure, which provides the lubricativeness.
Therefore, as shown in Japanese Patent KOKAI (LOPI) No. 11-131257, in the case of using the layered-structure solid lubricant as an injection particle, the impact at the time of colliding with the object to be treated incurs inter-layer peeling and causes the injection particles to be finely fragmentized so that many injection particles cannot be reused after one time-injection, which leads to an increase in a wear-out rate and hence in the cost of this kind of surface treatment.
As one example, when the layered-structure solid lubricant having an average particle diameter of 20 μm is injected onto the surface of the object to be treated as injection particles, the injection particles collected after the one-time injection have been fragmentized to pieces, each having an average particle diameter equal to or less than 10 μm. Even in the case of using any layered-structure solid lubricant as injection particles, it has been confirmed that when the injection particle is equal to or less than 5 μm in a diameter, an efficiency of the diffusion/penetration deteriorates, and when it is equal to or less than 1 μm in diameter, the efficiency thereof deteriorates extremely.
In addition, calculation of the expense of molybdenum disulfide required in the treatment based upon the wear-out rate of the injection particle in an attempt where a piston of an engine for an automobile is used as an object to be treated and the wear resistance layer is formed by use of molybdenum disulfide as an injection particle demonstrates that it is more costly by 100 yen or more per piston.
In a case where graphite, which is less expensive as compared with molybdenum disulfide, is used as an injection particle, the cost can be decreased to a low level. However, when graphite as carbon is fragmentized at the time of collision with the object to be treated to produce powder dust, there is a danger that powder dust fire or the like may possibly occurs. In addition, even when any one of the layered-structure solid lubricants is used, the powder dust produced due to the fragmentation pollutes and deteriorates the working environment.
In contrast, in the method described in Japanese Patent KOKAI (LOPI) No. 2000-282259 in which the solid lubricant-containing soft metal powder formed by mechanically contacting with the solid lubricant powder and the soft metal each other is used as injection particles, even in the case where the layered-structure solid lubricant is used as the solid lubricant powder, this solid lubricant powder is difficult to fragmentize because it is carried by the soft metal. Accordingly, it is thought that the problem with deterioration in the working environment caused by the powder dust is improved, as compared to the case of independently injecting the layered-structure solid lubricant.
However, according to the method described in Japanese Patent KOKAI (LOPI) No. 2000-282259 listed above, in order to obtain the injection particles, it is necessary that the solid lubricant powder and the soft metal are mechanically contacted with each other in advance to obtain the solid lubricant-containing soft metal powder, which is a complicated process.
Since the solid lubricant-containing soft metal powder obtained as described above is one obtained by carrying the solid lubricant powder on the particle surface of the soft metal, when the solid lubricant-containing soft metal powder is collided with the surface of the object to be treated, and the solid lubricant carried on the surface is adhered to the object to be treated, the amount of the solid lubricant equivalent to the one that has adhered to the object to be treated is lost from the surface of the solid lubricant-containing soft metal powder. Accordingly, using this repeatedly results in that the solid lubricant powder is transformed into one that cannot adhere to the object to be treated by a necessary amount.
Therefore, in the case of adopting the method described in Japanese Patent KOKAI (LOPI) No. 2000-282259, when the injection particle once injected is collected for reuse, unless the procedure is performed of causing the collected injection particles and the solid lubricant powder to mechanically contact with each other again, and of causing the solid lubricant powder to adhere to the surface of the soft metal, the layer with a stable quality cannot be formed.
Additionally, the layer formed with the solid lubricant or the like is used in place of the fluid lubricant such as oil or grease under the foregoing vacuum environment, but it is used under normal-pressure environment together with the fluid lubricant in many cases, and there are also many cases where it is provided for purpose of protecting the sliding portion from wearing in the event of the accidental loss of the oil layer in the sliding portion.
In the case of use of the layer with the fluid lubricant thus, it is preferable in a point of making generation of seizing etc. difficult all the more that minute concavities, which have the effect of an oil reservoir which makes it difficult that the oil layer loss occurs on the sliding portion of the object to be treated, are formed a lot.
However, in any method described above as the prior art, the concavity to achieve the effect of an oil reservoir like this cannot be formed simultaneously with the formation of the layer.
Thereupon, the present invention has been made for solving the problems in the above-mentioned prior art, and an object of the present invention is to provide a method for surface treatment of the sliding portion, which enables the surface treatment to be carried out economically for the sliding portion even in the case of using the layered-structure solid lubricant as an injection particle, the risk of pollution for the working environment, a powder dust fire, or the like is reduced, yet high lubricativeness is achieved, and moreover many concavities are formed on the sliding portion simultaneously with giving the lubricativeness.